Hi all, I know this is probably a stupid question. I am installing a second, larger, bilge pump to handle real water issues (Rule 2000), to supplement my primary pump which is a 500 gph. My question is this: Can I run the discharge from my smaller pump into the sink drain thru-hull? It would shorten the hose run by a great amount. I know that it would require keeping this thru-hull open all the time, but are there any other issues that I need to consider? Of course I know the larger pump would exit in the normal fashion.

We had a similar arrangement on a previous boat.. never really liked it though. As you've noted if the pump starts when the through hull is closed you have a hell of a mess.. and the bilge pump can overload the path overboard even with the valve open, and you'll end up filling the sink (and beyond). We added a valve immediately below the sink bowl to eliminate that issue.

But overall I don't think I'd install a set up like that again. Ours was done, I believe, for the same reasons you are considering - the short run of hose.

Montenido, I did exactly what you are contemplating and it works really well. The little Rule 600 gph pump with 3/4" hose discharges into the sink drain, and the 2000 gph through a dedicated through-hull. There is a horizontal check valve in the sink drain to prevent back-filling when heeled. I ran the 3/4" line up through a vented loop well above the water line.

I had a similar question earlier (see SailNet thread on gear-maintenance/44434-plumbing-2d-bilge-pump).

I had installed a second, high-capacity pump above the primary manual one and wanted to use a Y-connection to the existing through-hull. Idea was to avoid cutting a new hole in the hull.

Bottom line: I backed away from the Y-connection idea and instead emptied the new high-capacity, float-switch-activated pump right into the cockpit. Kept the smaller pump connected to the existing transom through-hull (above the waterline).

I love the idea. It's an alarm-clock solution. Now, if my feet get suddenly wet, I'm alerted to a Serious problem.

The thing that jumps out at me about doing this is that you'll have the bilge pump pushing water out of a submerged thru hull. Would it be possible for a siphon to occur back into the bilge?

Here's what would happen to me: I (or anyone else) leave the stopper in the galley sink. The bilge pump kicks on and pushes a little water up past the stopper. The bilge pump kicks off and the stopper reseals due to a little bit of water on top. Now we have a siphon between the thru hull and the bilge a couple of inches lower. This causes the bilge pump to come on again and pump the water back out. The bilge pump cycles like this until the battery is dead. I find my mast sticking up out of my slip.

Bottom line: I emptied the new high-capacity, float-switch-activated pump right into the cockpit. Kept the smaller pump connected to the existing through-hull. I love the idea. It's an alarm-clock solution. Now, if my feet get suddenly wet, I'm alerted to a Serious problem.

I like this idea too. Just two questions? How much water would make its way down below if the cockpit took a big wave? How high is the outlet above the bilge pump, it would be worth calculating the efficiency drop.

Bubb2 is correct. Although with a small pump operating at a reduced rate, you might get away with it for years, in the long run, connecting a bilge pump to an underwater thruhull is a disaster waiting to happen.

There are a few reasons for the problem, the most fundamental of which is that a thruhull into the water (under water) creates a tremendous restriction thereby severely reducing actual pumping rate regardless of the pumps' rated capacity - in essence, you would be wasting your money putting in a large capacity pump on a system that restricts it.

Exaggeration for effect - you try pumping 2000gpm into a hose already partially full (as it is below the waterline) and then into a body of water resisting flow by the head pushing up against the thruhull.

In addition, the possibility of a siphon is very real creating a problem that should never exist. simply installing a backflow valve is a bad idea as it can fail without you knowing it and can also easily become clogged without you knowing.

Just two questions? How much water would make its way down below if the cockpit took a big wave? How high is the outlet above the bilge pump, it would be worth calculating the efficiency drop.

1. Each of the two cockpit scuppers are three times the diameter of the inlet from the new, higher bilge pump. And that inlet through-hull is mounted in the footwell about five inches above the cockpit floor. So, if we were to get pooped and the footwell completely filled, there'd be some minutes in which the 1-1/8-in.-dia. hose back to the pump might act as a third drain. I'm betting that pump would impede flow into the bilge.

2. The cockpit outlet is about 4 ft. higher than the new bilge pump. No, I didn't do a calculation, but in tests there seems to be no problem with flow.

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